1. Field of the Invention
The present invention relates to a method of and apparatus for measuring the flatness of wafers that have not been subjected to donor-killer treatment.
2. Description of the Related Art
For the substrates of semiconductor devices, high-purity silicon single crystals are chiefly employed. One method of manufacturing such silicon single crystals is the Czochralski method (hereinbelow called the CZ method). In the CZ method, polycrystalline silicon is packed into a quartz crucible arranged in a chamber of a semiconductor single crystal manufacturing apparatus and the polycrystalline silicon is melted by heating it by means of a heater provided at the circumference of the quartz crucible, to form a molten liquid. A seed crystal mounted on a seed chuck is then dipped into this molten liquid, and the silicon single crystal is grown by raising the seed chuck while rotating the seed chuck and quartz crucible in the same or opposite directions. In this process, oxygen dissolves into the molten liquid due to reaction between the quartz crucible and the molten liquid, so the silicon single crystal that is pulled up contains about 10.sup.18 atm/cm.sup.3 of oxygen. This oxygen is present interstitially, so, on cooling the single crystal, oxygen donors are generated.
The oxygen donors are removed from the silicon single crystalline ingot by heat treatment after the slicing and grinding steps. After passing through these steps, the thickness and flatness of the wafer are measured.
When the flatness of donor-killer untreated wafers, that is, wafers that have not been subjected to donor-killer treatment, is measured using an electrostatic capacitance-type flatness measurement instrument, erroneous measurement, in which good products are identified as rejects, occur in a proportion of a few percent to a few tens of percent per lot. The reason for this is thought to be that, when the flatness is calculated using the detected values of wafer thickness, these detected values are in fact thicker or thinner than the actual wafer thickness, due to the presence of the interstitial oxygen in a donor-killer untreated wafer acting as a donor, the interstitial oxygen being distributed in irregular fashion. When an electrostatic capacitative flatness measurement instrument is employed, if the electrical resistance is different even for wafers of the same thickness, their thickness will be indicated as thicker or thinner than the true thickness. As an example, when two wafers of different electrical resistance but confirmed to be of the same thickness by a measuring instrument such as an electronic micrometer are measured using an electrostatic capacitative flatness measuring instrument, it is found that the wafer with the higher electrical resistance is indicated as being thinner than the wafer with the lower electrical resistance.
FIG. 4 shows a p-type as-grown wafer 4 in which the interstitial oxygen concentration is highly uneven such that the wafer 4 has portions H of higher electrical resistance (electrically neutral) and portions L of lower electrical resistance. In such a wafer, even if the thickness of the wafer is in fact uniform, it will erroneously be measured as thicker or thinner depending on the electrical resistance as shown in FIG. 5. Accordingly, precise flatness data cannot be obtained before the wafer has been subjected to donor-killer treatment. However, if reject products, whose flatness does not meet the required standard, are only picked out after they have been subjected to donor-killer treatment, this donor-killer treatment is a wasted step for the rejected products. Also, if wafers whose product specification does not require them to be subjected to donor-killer treatment still have to be subjected to donor-killer treatment in order to check their flatness, this likewise represents a wasted step.